1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus such as a digital copier, a printer, a plotter, and a facsimile using the optical scanning device.
2. Description of the Related Art
In the related art, with respect to an optical scanning device or an image forming apparatus using the optical scanning device, there is known an optical scanning device which deflects a light flux by a deflection unit such as an optical deflector, focuses the deflected light flux as micro-sized spot light on a scanning surface to scan the scanning surface in a main-scanning direction at a constant speed, and the optical scanning device is applied as a latent image writing unit or the like of an image forming apparatus such as a digital copier, a laser beam printer, a laser beam plotter, and a facsimile.
The optical scanning device scans the scanning surface such as a photoconductor by deflecting and reflecting, for example, laser light emitted from a laser light source by the optical deflector and, at the same time, writes an image on the scanning surface by intensity-modulating (for example, on or off) the laser light according to an image signal.
In addition, with respect to a color image forming apparatus, there is an image forming apparatus of obtaining a two-colored image, a multi-colored image, a full-colored image by using two or more sets of an optical scanning device and a photoconductor, and such an image forming apparatus is called a “tandem type image forming apparatus”.
As the tandem type image forming apparatus, for example, Japanese Laid-open Patent Publication No. 2006-323278 discloses a so-called opposite scan type optical scanning device of performing optical scanning on the left and the right by a single optical deflector, and this optical scanning device is well known.
In this manner, if the optical deflector is commonly used in a plurality of scanning surfaces, the number of optical deflectors is reduced, so that there is an advantage in that a small-sized, low-cost optical scanning device can be implemented.
In addition, in the optical scanning device used for the tandem type image forming apparatus, as measures for promoting low cost, Japanese Laid-open Patent Publication No. 2007-240590 discloses an oblique incidence optical system which allows a light flux to be incident on a deflecting reflection plane of an optical deflector with an angle in a sub-scanning direction.
In the oblique incidence optical system, a plurality of light fluxes are deflected and reflected on the respective deflecting reflection planes and are separated and guided to the corresponding scanning surfaces by a folding mirror or the like. In this case, a sub-scanning direction angle (hereinafter, referred to as an “oblique incidence angle”) of each light flux incident on the optical deflector is set as an angle where each light flux can be separated by the folding mirror.
Therefore, by thinning the optical deflector in the sub-scanning direction, a low-cost optical scanning device can be implemented.
In addition, in the case of using a polygon mirror for the optical deflector, high energy is not needed for high speed rotation, and it is possible to implement low power consumption and low noise.
On the contrary, in oblique incidence optical system, the sub-scanning direction shift amount varies due to the optical path length difference, so that there is the problem (hereinafter, referred to “scan line curve”) that the scan line on the sub-scanning is curved in the sub-scanning direction.
Therefore, in generally, the methods of removing the first order component of the scan line curve by the scan line tilt adjustment and removing the second order component of the scan line curve by the scan line bend adjustment are well known.
For example, in Japanese Laid-open Patent Publication No. 2007-240590, the scan line tilt is corrected by rotating the scanning lens about an axis parallel to the optical axis, and the scan line bend is corrected by bending the scanning lens so that the center of curvature of the scanning lens in the sub-scanning direction is adjusted and moved in the sub-scanning direction.
However, on the other hand, there is a so-called “sag” where, since the center of a polygon mirror is not coincident with a deflecting reflection plane, the reflection position of the light flux incident from the light source by the deflecting reflection plane varies according to the rotation of the polygon mirror.
When a main light beam of the deflected light flux which is deflected by the polygon mirror is perpendicular to the scanning surface, if the direction of the main light beam is defined as a reference line, the sag occurs asymmetrically with respect to the deflected light fluxes at the both sides of the reference line.
Therefore, asymmetry occurs in the optical path length difference of the light flux scanning the scanning surface, so that an asymmetric component occurs in the scan line curve. Particularly, in a wide angle-of-view optical system for implementing a small-sized optical scanning device demanded in the recent years, the sag is increased, so that the asymmetry of the scan line curve greatly occurs.
In addition, in the scan line tilt adjustment and the scan line bend adjustment described above, the third order component or the higher order components of the scan line curve, that is, asymmetric components are not completely removed, and when the latent images drawn by the respective light fluxes are superposed and visualized by the respective color toners, color shift occurs.
With respect to the correspondence to the asymmetric portion, Japanese Laid-open Patent Publication No. 2007-240590 discloses the correspondence to the asymmetric component of the scan line curve by configuring the scanning lens to be asymmetric in the main-scanning direction with respect to the optical axis.
However, in the above-described opposite scan type optical scanning device, in the opposite side, the scanning lens is used in a state of being inverted about the optical axis, and thus, there is a problem in that the optical scanning device cannot cope with the asymmetric component of the scan line curve and the optical performance of the optical system at the opposite side is greatly deteriorated.
In addition, Japanese Laid-open Patent Publication No. 2006-323278 discloses a technique where the number of folding mirrors is regulated so that it can be commonly used for scanning lenses having an asymmetric shape. However, since the number of arranged folding mirrors of the entire optical system is regulated, there is a problem in that the limitation of layout is high.
There is a need to provide an optical scanning device capable of correcting an asymmetric component of scan line curve with a simple configuration irrespective of layout while using common lenses.